This invention generally relates to fluorescent lamps which utilize a conductive coating on the envelope interior surface and, more particularly, to a method for processing such lamps which incorporate manganese-activated zinc silicate phosphor, in order to improve the performance of the zinc silicate phosphor.
The most common fluorescent lamp ballast used in the United States is the so-called rapid-start ballast which is adapted to operate two fluorescent lamps each rated at 40 watts input. Such lamps normally utilize an inert gas filling comprising about two torrs of argon. By replacing the argon with a gas filling comprising about two torrs of a mixture of 80-85 volume percent krypton and 20-15 volume percent neon or 20-15 volume percent argon, the efficacy of the lamp can be slightly improved with a simultaneous decrease in wattage consumption. For example, such a change will typically increase the lamp-operating efficacy by about 6 to 7% percent while simultaneously decreasing the wattage consumption for each lamp from 40 watts to 34 watts. These figures are given only by way of example and are subject to some variations depending upon various design modifications. Such lower wattage lamps can be substituted for the existing higher wattage lamps and thus represent a substantial energy savings.
One of the problems encountered with such a modified inert gas fill is that the lamps are somewhat difficult to start from the rapid-start ballasts. To overcome this problem, it has been found desirable to coat the inner surface of the lamp envelope with a transparent tin oxide conductive coating. This in turn causes phosphor adherence problems. To overcome these adherence problems, it has been found desirable to overcoat the tin oxide conductive coating with a film of sub-micron-size aluminum oxide. The phosphor is then coated over this film of aluminum oxide and the resulting lamp readily starts and operates very efficiently at a lower wattage.
U.S. Pat. No. 3,858,082, dated Dec. 3, 1974 to Thornton, discloses various three-component phosphor blends which can be used in fluorescent lamps in order to provide both good color rendition of illuminated objects and a high light output. One embodiment of a phosphor blend which is disclosed in this patent uses apatite-structured strontium chlorophosphate activated by divalent europium as a blue-emitting phosphor component, manganese-activated zinc silicate phosphor as a green-emitting phosphor component, and yttrium oxide activated by trivalent europium as a red-orange emitting phosphor component. The relative proportions of these components can be varied to provide the lamp with a predetermined correlated color temperature, and the most popular color temperature for these lamps is about 3,000.degree. K. The overall performance of such lamps is excellent, but on occasion the green-emitting phosphor component displays a relatively rapid depreciation of light output, particularly in the vicinity of the electrodes, which causes a color shift to occur. This can be considered objectionable from an aesthetic standpoint and the alumina precoat appears to aggravate this problem.
It is well known to add a small amount of antimony oxide to a silicate-type phosphor in order to improve the performance thereof, as disclosed in U.S. Pat. No. 2,607,014 dated Aug. 12, 1952 to Roy et al. In U.S. Pat. No. 3,348,961 dated Oct. 24, 1967 to Ropp et al., is disclosed adding a small predetermined amount of finely divided antimony oxide to the paint used for coating manganese-activated zinc silicate, in order to improve the performance of the fluorescent lamp which incorporates the zinc silicate phosphor.
The internationally accepted procedure for standardizing and measuring the color-rending properties of light sources is set forth in the publication of The International Commission on Illumination, identified as Publication CIE No. 13(E-1.3.2) 1965. More recently, a color-preference index has been proposed for rating the performance of the light sources in accordance with what the normal observer considers to be the preferred coloration for familiar objects. This color preferance index (CPI) is summarized in the Journal of the Illuminating Engineering Society, pages 48-52 (Oct. 1974) article entitled "A Validation of the Color-Preference Index" by W. A. Thornton.